| Seagrass beds are declining around the world as water quality is compromised by agricultural runoff and coastal development. Elevated nitrogen levels stimulate epiphyhtic algal growth which in turn reduces the amount of light reaching the seagrass. At the same time, epiphytes are an important food source, and enhanced epiphytic algal production may increase the flow of organic matter to higher trophic levels in seagrass food webs.; To determine the extent to which nutrient enrichment alters the contribution of seagrass and epiphyte production to the food web, an isotope manipulation technique was developed and used to track the flow of organic matter in seagrass beds. The manipulations were conducted at Horn Island in Mississippi Sound and at Big Lagoon, Perdido Key in Halodule wrightii beds exposed to relatively high and low water currents, respectively. A 14N-enriched fertilizer was used to simulate eutrophication, and the functional response of the seagrass and epiphytes was measured. In addition, primary producer and consumer stable isotope compositions (delta 15N, delta13C, and delta34S) were determined to identify trophic relationships.; The functional response of the primary producers to nutrient enrichment was similar at both sites. The fertilization treatments did not affect seagrass production or biomass measured after 2, 4, and 8 weeks, but they did cause a decrease in leaf C/N ratios at Horn Island. Similarly, epiphyte production, biomass, and photopigment composition did not change in response to nutrient enrichment.; The isotope manipulation generally was successful in labeling the seagrass and epiphytes. During the first year, water column fertilization generated distinct delta15N values for the seagrass and epiphytes, whereas distinct values for these primary producers were achieved through sediment fertilization in the second and third years. Although the isotope manipulations did not yield definitive conclusions regarding the relative contribution of the primary producers to consumer diets by itself, analysis of nitrogen, carbon, and sulfur isotopes suggests that algae are the dominant food source for the majority of consumers in these seagrass communities. In addition, the isotope manipulation appears to have potential for addressing questions of habitat use and movement of consumers. |
